Robert Provine’s book Laughter: A Scientific Investigation has a very interesting chapter on contagious laughter. This curious phenomenon has long been exploited in such items as laugh boxes and musical laugh records, as well as being central to laugh tracks (from Ancient Rome to modern TV) and churches of “holy laughter”.

Contagious laughter is, of course, also an everyday occurrence, spreading directly from person to person in normal interaction. But even this activity can become abnormal, when for instance instead of dying down it persists and spreads over a wide area, as happened in the Tanganyika laughter epidemic (though it wasn’t just laughter).